Light: Science & Applications 7, e17172 (2017)

Enhanced light-matter interactions are the basis of surface enhanced infrared
absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic
materials and their capability to focus light to nanoscale spot sizes. Phonon
polariton nanoresonators made of polar crystals could represent an interesting
alternative, since they exhibit large quality factors, which go far beyond those of
their plasmonic counterparts. The recent emergence of van der Waals crystals
enables the fabrication of high-quality nanophotonic resonators based on
phonon polaritons, as reported for the prototypical infrared-phononic material
hexagonal boron nitride (h-BN). In this work we use, for the first time, phononpolariton-
resonant h-BN ribbons for SEIRA spectroscopy of small amounts of
organic molecules in Fourier transform infrared spectroscopy. Strikingly, the
interaction between phonon polaritons and molecular vibrations reaches
experimentally the onset of the strong coupling regime, while numerical
simulations predict that vibrational strong coupling can be fully achieved.
Phonon polariton nanoresonators thus could become a viable platform for
sensing, local control of chemical reactivity and infrared quantum cavity optics
experiments.